Although to many it may seem incredible, yet it is not so rare as might be supposed, that the adult pair may be married for years, and the wife still physically a virgin owing to neither of the pair knowing that penetration must be effected. Amazing as it may seem, four or five such cases, all of intelligent apparently average people, have come to my direct knowledge in the course of one year alone. Another cause, less extreme, is due to the woman making full entry difficult or impossible by not taking up a proper position during union. (See also p. 114.) In such cases a knowledge of the true details involved may speedily bring the desired conception. These very simple suggestions are of the kind often overlooked by the medical specialist, to whom a woman goes tremblingly asking if she is abnormally formed in any way, because she does not get the children she so much longs for. Such advice, of course, will apply only to people who are essentially normal and without deformity. For more serious obstructions to parenthood, the pair, and not the woman only, should seek medical advice. Appendix NOTE 1.--(See p. 24.) For suffering and even death of unmated females, see _e.g._ MARSHALL, in _Quarterly Journal Microscopical Society_, Vol. 48, 1904, p. 323. PARSONS, in _British Medical Journal_, October, 1904. NOTE 2.--(See p. 31.) A frequent mistake (made even by gynæcologists) is to confuse menstruation with the "period of desire," which is generally called "heat" in animals. Even in the most authoritative recent text books, such phrases as "heat and menstruation" are very common, thus coupling heat and menstruation as though they were equivalents, while the older books quite explicitly look on the menstrual period in women as corresponding to desire of "heat" in animals. This error has even been repeated very recently in the _Proceedings_ of the Royal Society of Medicine.[13] [13] See Dr. Raymond Crawfurd's mistaken statement that "the identity of oestrus, or 'heat' in the lower animals and of menstruation in the human female, admits of no doubt." P. 62 _Proc._ Roy. Soc. Medicine, vol. 9., 1916. Some physiologists have studied this subject in several of the higher animals, and now realise that the time of desire is physiologically distinct from the phase which is represented by menstruation in women. It seems to be fairly well established that in women menstruation is caused by an internal secretion of the ovaries (_c.f._ p. 61), and is not directly due to ovulation, though it must have some connection with it.[14]